Case Study: Back Pressure turbine in place of PRV

Area of Improvement:

Steam was being produced from 2 different boilers of 20TPH at 17 barg pressure in a chemical process industry. Average load of site was 30TPH, High pressure steam of 17 barg was transferred to different user blocks for end use. Steam was majorly used for process heating at 3 barg pressure.

Observation:

High pressure steam was transferred to various production blocks majorly for heating application at different pressure and temperatures. Major part of steam was being used at 3 barg pressure at production block A. Pneumatic operated PRV was in use for reducing steam pressure to 3 barg. After PRV station steam was 2-3 Deg C above saturation temperature so desuperheating station was not in place. The location of PRV was just before the user block, after PRV steam was used in different reactors, columns and heat exchangers. Kinetic energy of high pressure turbine was being lost at PRV station.

Energy Conservation Measure:

Back pressure turbine of 400KW was installed to generate power of 380KWhr from the available kinetic energy of pressure drop across turbine. It was easy to calculate power generated from turbine, Normal efficiency of back pressure turbine for saturated steam turbine is range of 80-90% . Some of the manufacturer reduces steam quality ie dryness of steam to 94-96% to increase power generation from turbine and remove condensate from downstream pipeline. As energy is converted in turbine from one form to another, cost of energy equivalent of steam to be removed from the power gain achieved through turbine.

Different manufactures uses different turbine speed to generate power as the speed to turbine increases there is increase in the efficiency of turbine but the metallurgy and cost of system increases accordingly.

Payback in the back pressure turbine depends on the steam and power cost of site, typically payback of turbine above 150KW is in range of 1-2 years.

Limitation:

Consistent pressure and flow is important for the operation of back pressure turbine. As turbine was installed in the process selection of turbine is very important. Idle selection of turbine should not be at average load but it should be such operation of turbine must be at full load condition. Similar precaution to be taken while deciding inlet and outlet pressure of turbine

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